In a waste water treatment process, one important process step is to oxygenate the waste water in order to promote biological consumption and removal of dissolved and suspended waste material. Aeration/oxygenation is performed in a natural waste water impound, such as lakes, ponds, or the like, or in a man-made waste water impound, such as tanks, basins, racetracks, lagoons, or the like, by using submerged gas distribution systems comprising a large number of diffuser members, also called aeration devices. In most cases the waste water impound is made of metal of concrete. Such gas distribution systems are also known as gas distribution grids. Compressed gas, such as air or other treatment gas containing some form of oxygen, is supplied to the submerged gas distribution system and then the compressed gas is diffused/discharged into the liquid by means of said numerous diffuser members as small bubbles. As these bubbles rise buoyantly through the liquid, oxygen in the bubbles dissolves into the waste water. Bacteria are supplied to the waste water in the treatment process in order to consume the waste in the liquid, and the life processes of bacteria is supported by the oxygen. Other treatment gases, not necessarily containing oxygen, may be distributed through the gas distribution system for other purposes, such as for cleaning the small discharge openings of the diffusers.
In many applications the gas distribution system is liftable, i.e. removable from the basin, in order to facilitate the service and cleaning of the gas distribution system and the basin, respectively. In order to make the anchorage as well as the removal of the gas distribution system easier, prior art gas distribution systems according to the introduction above have been developed, i.e. gas distribution systems as the one disclosed in DE 36 24 580 and DE 35 14 028. Older gas distribution systems needed a lot of additional weights to outweigh the large buoyancy of the gas distribution system, however the extra weights also made the gas distribution system more cumbersome to remove from the basin. Gas distribution systems according to DE 36 24 580 and DE 35 14 028 comprise an internal space delimited by an open ended inner pipe, which internal space is filled with liquid during lowering in order to reduce the buoyancy, during hoisting the liquid is automatically emptied from the internal space when the gas distribution system is hoisted above the liquid surface.
However, known low buoyancy gas distribution assemblies according to the introduction, and shown in DE 36 24 580 and DE 35 14 028, are only made for tube diffusers of limited length. Thereto, it is time consuming to arrive at the exact corresponding length of the inner pipe and the outer pipe, which is crucial in order to obtain a faultless and leakage free tube diffuser of the known type.